Chickens have a natural pecking order that, once established, can wreak havoc once a new hen or roo is introduced. That's why when you bring your new bird(s) home, you need to take extra care during the integration process to make sure everything goes smoothly. I've done this twice and made my own method by compiling methods from a variety of sources that I hope can be useful! Keep in mind that, if possible, you should introduce more than one bird at a time so one is not targeted. This goes especially for bantams because they are so much smaller. Until step five, give your newbies different sleeping quarters than the main flock. This will cause a lot of stress to you and both sides of the flocks, and this will take patience!
1. Put the newbie(s) in a cage where the flock can "see", but not "touch".
The first step during the integration process is to get some sort of cage for your newbies to be exposed to the flock but secured safely behind the bars if one of the more dominant birds decides to get rough. Make sure you provide them with food and water. I also like to make a "safe corner" where the newbie(s) can huddle so they have somewhere to run. Although any new hens will probably be too stressed to lay at this point, it doesn't hurt to add bedding too.
You'll know when this step is done because soon your main flock will lose interest and will ignore the newbies. You've completed the first step!
2. Introducing the newbies
Next, I like to place the newbies in a separate run and put in one or two of my more docile birds. Keep in mind there will be pecking no matter what, as that's how the pecking order works. Always supervise this step; because this is their first time meeting without a wall in between them, you never know what might happen. Keep doing this until, like the first step, the newbies and the docile birds don't mind each other and go on their chicken-y business without much bugging each other.
3. Getting to know the meaner birds
When you've gotten to this step, but in the newbies with either the same group as before plus a few (it really depends on how big the flock is) or put them with the rest of the flock. I have a flock of five and threw in my one newbie with two docile birds and then automatically with the rest of the flock once they settled. Supervise this too; more dominant birds will feel threatened and try to show the newbies who's boss. This is normal unless you see blood. If you see blood during any of these steps, go back to the last step. Eventually, the newbies will be slowly accepted into the flock.
4. Being with each other full time
Once you're confident that your birds are nearly integrated, put them with your flock as you normally do. Just make sure you have the option to check on them every now and then. I free-range mine in my backyard and when I do this I like to keep windows open on that side of the house so I can hear any squawking.
5.Into the coop
Soon you'll be able to put the newbies in the coop, congratulations! If they don't go into the main coop on their own, put them on the roost but on the opposite side of the roost to main flock. Do this when it's already getting dark and your birds are getting ready to sleep so they don't have energy to peck.
The thing about this, though, is that you'll have to get up early to let them out in case the newbies get trapped an enclosed area. Depending on the time of year, 5:45-6:00 should be good; the key is to go as it's getting brighter but not the crack of dawn. Over time, you can make this alarm later and eventually put in the birds earlier and earlier.